The U.S. is heading toward one of the worst West Nile seasons since 1999, when the virus first appeared in this country, government health officials said on Wednesday.

The tally of cases and deaths from the virus spiked again this week: so far, there have been a total of 1,118 West Nile infections and 41 deaths reported to the Centers for Disease Control and Prevention (CDC), up from 693 cases and 26 deaths the week before. In an average year, fewer than 300 cases are reported by mid-August.

“The number of West Nile cases in people has risen dramatically in the last few weeks and indicates that we are in one of the biggest West Nile virus outbreaks we have ever seen in this country,” said Dr. Lyle Petersen, director of the CDC’s Division of Vector-Borne Infectious Diseases, in a phone conference on Wednesday

More cases are expected to emerge, since peak season for the disease has only just begun, in mid-August, and symptoms of infection typically don’t show up for three to 14 days after a person has been infected through a mosquito bite.

As of Tuesday, 47 states had reported the presence of West Nile virus in birds, mosquitoes and humans, and 38 states have reported human cases of the disease. Approximately 75% of the cases are coming from the southern states Texas, Mississippi, Louisiana, South Dakota and Oklahoma — and about half of all human cases are from Texas alone.

Among people infected with West Nile virus, 80% will experience no symptoms. The other 20% will come down with symptoms of West Nile fever, which range from aches and pains to swollen lymph glands and a rash. In about 1% of cases, people develop neuroinvasive West Nile disease, which may cause inflammation of the brain and its surrounding tissues; symptoms may include high fever, headache, tremors and muscle weakness, and even paralysis, coma and death.

Of the reported cases, 629 are classified as neuroinvasive and 489 as non-neuroinvasive. In the phone conference, the CDC’s Dr. Petersen said the relatively higher numbers of neuroinvasive illnesses could be due to better reporting and diagnosis: testing for the more severe form of the disease is very precise, and people infected with milder forms of the disease may not bother contacting their doctor. The CDC estimates that only 2% to 3% of infections are reported.

Why is this year’s outbreak so bad? You can blame the weather. A mild winter allowed more West Nile-carrying mosquitoes to survive, while an early, rainy spring let them thrive; high temperatures this summer then increased mosquito populations by speeding up their life cycle, and spurred development of the virus in their bodies.

“It is a complicated ecological cycle among people, animals and birds, but one observation is that in the U.S. and elsewhere, hot weather seems to promote West Nile virus outbreaks,” said Petersen. The CDC has not determined whether the virus has mutated.

For more on how to keep yourself protected from West Nile, check out our coverage on what you need to know about the disease.